It is generally recognized that a sensor-fault detection circuit of this type is required in any instance where such sensors are to be employed in conjunction with safety-related devices. One particularly well known area where this applies is when these sensors are utilized in a vehicle anti-locking brake system. As a rule, inductive-type sensors are employed in such vehicle anti-lock brake system to measure the rotational speed of the wheels on such vehicle. In this application, it is obvious that these sensors are subjected to a number of undesirable conditions. These undesirable conditions include dirt, moisture and rather high vibration at times. Therefore, these sensors will exhibit a relatively high susceptibiity to failure. Consequently, it is imperative that any failure occurring in such a sensor be detected as quickly as possible. Such early detection being desirable in order to substantially eliminate a serious malfunctioning in the control of such anti-lock brake system.
A fault detection circuit for a vehicle anti-lock brake control system has been taught in the prior art. See, for example, German patent publication DE-OS 31 26 102. The fault detection circuit taught in this particular reference monitors the anti-lock brake control system while the vehicle is stopping. In order to accomplish this, a safety circuit disconnects each of the respective wheel speed sensing devices. After the wheel speed sensors are disconnected by the safety circuit, the same test signals are fed into one pair of control channels each. In this system, identical test signals pass through each of the two pairs of control circuits. At this point the fault detection circuit will compare the signals which are generated at the output end in each pair of such control circuits. During this comparison, the fault detection circuit makes the assumption that in the case of a properly functioning control circuit the generated signals will be the same at the respective points. However, in this known fault detection circuit, as soon as a non-agreement of these generated signal sequences can be detected such fault detection circuit causes a warning or switch signal to be produced. A check of the wheel speed sensors proper does not take place.
A fault detection circuit for a skid control system is also taught in German patent publication DE-OS 32 34 637. In this particular prior art fault detection circuit, the wheel speed sensors are also monitored with an astable multivibrator while the vehicle wheels are stopped. When the wheel speed sensor is connected and functioning properly in this system, a low-level natural frequency is produced. This low-level natural frequency is interrupted, however, in the case of a sensor malfunctioning. Such interruption occurs either by a short-circuit or by disconnection. Consequently, this will result in a different signal sequence as compared to the other operative wheel speed sensor circuits. This different signal sequence is perceived as a malfunction in this fault detection circuit. A major drawback with this fault detection circuit is that it is relatively complicated.